1. Field of the Invention
This invention relates to a regenerative system for burning off the particulates trapped in particulate trap filters which collect them from the exhaust gas of diesel engines and the like.
2. Prior Art
In a diesel engine there has been conventionally provided an exhaust gas purification device in the exhaust pipe of the engine to prevent particulates produced by the combustion operation of the engine from being emitted into the air as black smoke. When a particulate trap filter housed in a cylindrical housing of the exhaust gas purification device is full of the particulates due to a prolonged use and the like, the back pressure of the filter will increase to drop of output power of the engine. To prevent this from happening, there is provided a burner at the sectional center of the exhaust pipe in the art to remove particulates such as carbon particulates accumulated on the filtering surface of the particulate trap filter by burning them off periodically or when the back pressure of the filter becomes higher than a fixed value. However, with the arrangement of the conventional burner mentioned above, when the flame from the burner is applied to the particulate trap filter as it is, since the flame flows more to the central portion of the filter than to the periphery thereof, the central portion tends to reach an elevated temperature and then the particulates like carbon particulates spread all over the filter surface will not burn uniformly. In addition, because of the localized heating of the particulate trap filter, the filter is likely to have damage including cracks between the peripheral and central portions thereof caused by thermal stresses set up therein due to temperature differential, or to be fused when the elevated temperature reaches the melting point.
To solve the problem above mentioned, there has been proposed in publications including Japanese Utility Model Laid-Open Publication No. 15713 of 1989 a particulate trap filter regenerative system in which a flame flow control member for diffusing the flame from the burner to the periphery inside the exhaust pipe is set up between the particulate trap filter and the burner. Because the flame flow control member is disposed at the location opposite to the end of the particulate trap filter, however, the flame flow control member multiplies its effect to diffuse the flame flow to the periphery inside the exhaust pipe together with a natural convection of the flame flow which occurs when the speed of the flame flow from the burner slows down, and hence the flame flow tends to propagate toward the upper part inside the exhaust pipe to result in an elevated temperature higher by about 400.degree. C. or more at the upper peripheral portion of the particulate trap filter than the temperature at the central portion thereof. When the flame from the burner is injected in a slant manner with respect to the exhaust pipe, the flame will hit the lower part thereof and then propagate along the same. Thereupon, the lower peripheral portion of the particulate trap filter also reaches an elevated temperature higher by about 400.degree. C. or more than the temperature at the central portion thereof as mentioned above.
Accordingly, because of the localized distribution of the flame with the variation in flow speed of the flame or in accordance with the injection angle thereof during an engine idling operation in which the flow of a exhaust gas becomes minimum for example, even the particulate trap filter regenerative system with the flame flow control member mentioned above cannot burn off the particulates uniformly. As a result, there likely arise problems such that the back pressure gradually increases to drop of the output power of the diesel engine or damage including cracks or fusing of the particulate trap filter caused by localized heating thereof occur.